To really understand the strength of a metal, we’ve go to take a look deep inside the metal as a force is applied to it.
We have seen that when a force is applied to an object, a stress is induced. When a metal is stressed, it reacts by deforming. Deformation is a change in the length or shape of an object and it can happen in two main ways: Elastic deformation and plastic deformation.
To see the difference between these two types of deformation, we need to look closely at the metal. We need to look beyond the metal grains down to the individual atoms that make up the structure of the metal.
Remember that these atoms are held together atomically by metallic bonds. These bonds are slightly flexible so when the atoms are pulled apart or punished together, the bonds will act like springs. In fact, they act exactly like springs because it is this action that makes a metal spring work. When you pull on a spring, what you are feeling is the metallic bonds stretching apart and trying to pull the metal atoms back together.
This type of deformation is called elastic deformation. You may not have realized it before, but this happens in all metal objects, not just springs. If a metal is only deformed elastically, it is able to snap back to its original condition without being permanently changed.
Now if a metals is stretched too far, something else entirely happens. Let’s look at those atoms again. If a metals is stressed to a point called the yield point, the metallic bonds will be stressed so much that they can no longer hold the atoms in position. Groups of atoms will begin to slide past each other and the material will be permanently changed. Even if the load is removed, the atoms will never return to their original position. This type of deformation is called plastic deformation.
When we say something can deform plastically we mean that it can be shaped or formed by bending or stretching it. Plastic deformation is useful in metals because it allows them to be bent, rolled, or drawn into useful shapes.
Plastic deformation can even make a metal stronger. When the metal atoms begin to slip past each other it becomes more and more difficult for them to keep slipping. This helps prevent the metal from deforming plastically. This phenomenon, known as work or strain hardening actually makes the metal stronger, but it also becomes less ductile.